In a magnetic media, digital information (expressed as combinations of “0's” and “1's”) is written on tiny magnetic bits (which themselves are made up of many even smaller grains). When a bit is written, a magnetic field produced by the disc drive's head orients the bit's magnetization in a particular direction, corresponding to either a 0 or 1. The magnetism in the head in essence “flips” the magnetization in the bit between two stable orientations.
The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of remanent coercivity (Hr), magnetic remanance (Mr), coercivity squareness (S*), signal-to-medium noise ratio (SMNR), and thermal stability of the media. In particular, as the SMNR is reduced by decreasing the grain size or reducing exchange coupling between grains, it has been observed that the thermal stability of the media decreases. Thus, various compositions and structures of a magnetic layer, materials for a nonmagnetic base layer, and the like have been proposed including the use of a “granular magnetic layer.” A “granular magnetic layer” is a magnetic layer in which a nonmagnetic substance that is preferably nonmetallic, such as an oxide or nitride, surrounds a periphery of ferromagnetic crystal grains. A possible explanation for the reduction in noise in a granular magnetic layer could be due to an improved separation between magnetic grains by the nonmagnetic, nonmetallic grain-boundary phase that physically separates the magnetic grains from one another to weaken a magnetic interaction among the magnetic grains, thereby hindering zigzag magnetic domain walls from being formed in recording-bit transition regions.
The ease of magnetization or demagnetization of a magnetic material depends on the crystal structure, grain orientation, the state of strain, and the direction of the magnetic field. The magnetization is most easily obtained along the easy axis of magnetization but most difficult along the hard axis of magnetization. A magnetic material is said to posses a magnetic anisotropy when easy and hard axes exist. On the other hand, a magnetic material is said to be isotropic when there are no easy or hard axes. A magnetic material is said to possess a uniaxial anisotropy when the easy axis is oriented along a single crystallographic direction, and to possess multiaxial anisotropy when the easy axis aligns with multiple crystallographic directions.
“Anisotropy energy” is the work against the anisotropy force to turn magnetization vector away from an easy direction. For example, a single crystal of iron, which is made up of a cubic array of iron atoms, tends to magnetize in the directions of the cube edges along which lie the easy axes of magnetization. A single crystal of iron requires about 1.4×105 ergs/cm3 (at room temperature) to move magnetization into the hard axis of magnetization from an easy direction, which is along a cubic body diagonal. Important magnetic properties, such as coercivity (Hc), remanent magnetization (Mr) and coercive squareness (S*), which are crucial to the recording performance of the Co alloy thin film for a fixed composition, depend primarily on its microstructure. For thin film longitudinal magnetic recording media, the desired crystalline structure of the Co and Co alloys is HCP with uniaxial crystalline anisotropy and a magnetization easy direction along the c-axis is in the plane of the film. Moreover, longitudinal media is often sputtered on textured substrate to further align magnetic easy axis along the textured lines. The better alignments of easy axis in the plane of the film and along texture lines lower noise of the longitudinal recording media. For very small grain sizes coercivity increases with increased grain size. As grain size increases, noise increases. There is a need to achieve stable recording media without the increase in noise associated with large grains. To achieve a low noise magnetic medium, the Co alloy thin film should have uniform small grains with grain boundaries that can magnetically isolate neighboring grain diameters. This kind of microstructure and crystallographic texture is normally achieved by manipulating the deposition process, or most often by the proper use of an underlayer.
Efforts are continually being made to increase the areal recording density, i.e., the bit density, or bits/unit area, and signal-to-medium noise ratio (SMNR) of the magnetic media. To continue pushing areal densities and increase overall storage capacity, the data bits must be made smaller and put closer together by making the magnetic grains storing data bits smaller. However, as the grains become small, there are two problems.
First, the magnetic energy holding the grain in place may become so small that thermal energy may cause it to demagnetize over time. This phenomenon is known as superparamagnetism. To avoid superparamagnetic effects, one has to increase anisotropy of the material used in the media, but the available writing fields limit the anisotropy increase.
In conventional magnetic recording media CoCr alloys are deposited on pre-heated substrate to increase mobility of the atoms landing on the substrate and therefore improve segregation of Cr-rich phase in grain boundary. Moreover, pre-heating of substrate is also required to establish Cr(200)/Co(11-20) texture required for traditionally used longitudinal recording media. In particular, for the conventional CoCr-based metallic magnetic films, it is essential to increase the temperature of the substrate to 200° C. or higher during film formation in order to diffuse Cr in grain boundary. Furthermore, to increase the recording density and reduce the noise of a magnetic recording medium, an inter-grain magnetic interaction should be weakened by facilitating segregation in the magnetic layer, and a crystal orientation of the CoCr-based ferromagnetic crystal grains should be controlled. Specifically, the c-axis of the hexagonal close-packed ferromagnetic crystal grains of longitudinal media should be oriented in a film surface. For this purpose, in a case of the prior art metallic magnetic layers, the crystal orientation of the magnetic layer is realized by controlling a structure and crystal orientation of the nonmagnetic base layer.
For further improvement of the magnetic properties and reduction of noise, this invention proposes a novel process for fabricating a novel longitudinal granular oxide media. Granular oxide media of this invention have sharper transitions between non-magnetic (grain boundary) and magnetic (magnetic grains) regions than diffusion-segregated media that is grown at elevated temperatures. For this reason magnetic grains in granular media have higher magnetic saturation, Ms, anisotropy and narrower switching field distribution, while maintaining low inter-granular exchange coupling.